U.S. patent number 10,498,458 [Application Number 15/342,975] was granted by the patent office on 2019-12-03 for optical n-level quadrature amplitude modulation (nqam) generation based on phase modulator.
This patent grant is currently assigned to Futurewei Technologies, Inc.. The grantee listed for this patent is Futurewei Technologies, Inc.. Invention is credited to Yu Sheng Bai, Xiao Shen, Yangjing Wen, Xueyan Zheng.
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United States Patent |
10,498,458 |
Zheng , et al. |
December 3, 2019 |
Optical N-level quadrature amplitude modulation (NQAM) generation
based on phase modulator
Abstract
An optical modulator for generating quadrature amplitude
modulation (nQAM) and phase-shift keying (nPSK) signals with
tunable modulation efficiency. The modulator includes a controlling
circuit for adjusting the modulation efficiency or modulation depth
of the modulator by controlling the direct current (DC) bias.
Inventors: |
Zheng; Xueyan (Andover, MA),
Bai; Yu Sheng (Los Altos Hills, CA), Shen; Xiao (San
Bruno, CA), Wen; Yangjing (Cupertino, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Futurewei Technologies, Inc. |
Plano |
TX |
US |
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Assignee: |
Futurewei Technologies, Inc.
(Plano, TX)
|
Family
ID: |
57517965 |
Appl.
No.: |
15/342,975 |
Filed: |
November 3, 2016 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20170134096 A1 |
May 11, 2017 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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62250171 |
Nov 3, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B
10/541 (20130101); H04B 10/5051 (20130101); H04B
10/50575 (20130101); H04B 10/5561 (20130101); H04B
10/5055 (20130101); H04B 10/5161 (20130101) |
Current International
Class: |
H04B
10/00 (20130101); H04B 10/556 (20130101); H04B
10/516 (20130101); H04B 10/50 (20130101); H04B
10/54 (20130101); H04J 14/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Kiuchi, H., el al., "High Extinction Ratio Mach-Zehnder Modulator
Applied to a Highly Stable Optical Signal Generator," IEEE
Transactions on Microwave Theory and Techniques, vol. 55, No. 9,
Sep. 2007, pp. 1694-1972. cited by applicant .
T. Sakamoto, el al., "50-Gb/s 16 QAM by a Quad-Parallel
Mach-Zehnder Modulator," Optical Communication--Post Deadline
Papers, 33rd European Conference, 2007, 2 pages. cited by applicant
.
M. Serbay, et. al., "Implementation of Differential Precoder for
High-Speed Optical DQPSK Transmission," Electronics Letters, vol.
40, Issue 20, Sep. 20, 2004, pp. 1288-1289. cited by applicant
.
Y. Ehrlichman, el al, "Improved Digital-to-Analog Conversion Using
Multi-Electrode Mach-Zehnder Interferometer," Journal of Lightwave
Technology, vol. 26, No. 21, Nov. 1, 2008, pp. 3567-3575. cited by
applicant .
Foreign Communication From a Counterpart Application, PCT
Application No. PCT/CN2016/060381, English Translation of
International Search Report dated Feb. 10, 2017, 6 pages. cited by
applicant .
Foreign Communication From a Counterpart Application, PCT
Application No. PCT/CN2016/060381, English Translation of Written
Opinion dated Feb. 10, 2017, 10 pages. cited by applicant .
Ehrlichman, Y., et al., "A Method for Generating Arbitrary Optical
Signal Constellations Using Direct Digital Drive" Journal of
Lightwave Technology, vol. 29, No. 17, Sep. 1, 2011, pp. 2545-2551.
cited by applicant.
|
Primary Examiner: Dobson; Daniel G
Attorney, Agent or Firm: Conley Rose, P.C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority to U.S. Provisional Patent
Application Ser. No. 62/250,171, by Xueyan Zheng et al., filed Nov.
3, 2015, titled "Optical N-Level Quadrature Amplitude Modulation
(nQAM) Generation Based on Phase Modulator," and incorporated
herein by reference as if reproduced in its entirety.
Claims
What is claimed is:
1. An apparatus for generating a modulated data signal, the
apparatus comprising: a precoder configured to code data for
modulation and to output a plurality of electrical data signals; a
Mach-Zehnder modulator (MZM) coupled to the precoder, the MZM
configured to receive a first electrical data signal of the
plurality of electrical data signals and to modulate an optical
input signal to produce a first modulated signal according to the
first electrical data signal; a phase modulator (PM) coupled to the
precoder and to the MZM, the PM configured to: receive a second
electrical data signal of the plurality of electrical data signals,
receive the first modulated signal from the MZM, and modulate the
first modulated signal to produce a second modulated signal
according to the second electrical data signal; and a controlling
circuit coupled to the PM, the controlling circuit configured to
adjust a direct current (DC) bias of the PM, wherein the MZM is
configured to generate 0 and .pi. phase changes, the PM is
configured to generate 0 and +.pi./2 phase changes, and the second
modulated signal is a 4-level quadrature amplitude modulation
(4QAM) signal.
2. The apparatus of claim 1, further comprising a laser diode
configured to transmit the optical input signal, and wherein the
laser diode is coupled with the MZM.
3. The apparatus of claim 1, wherein the MZM and PM comprise
silicon photonics (SiP) materials.
4. An apparatus for generating a modulated data signal, the
apparatus comprising: a precoder configured to code data for
modulation and to output a plurality of electrical data signals; a
Mach-Zehnder modulator (MZM) coupled to the precoder, the MZM
configured to receive a first electrical data signal of the
plurality of electrical data signals and to modulate an optical
input signal to produce a first modulated signal according to the
first electrical data signal; a phase modulator (PM) coupled to the
precoder and to the MZM, the PM comprising a first PM segment
coupled with a second PM segment, the PM configured to: receive a
second electrical data signal of the plurality of electrical data
signals, receive the first modulated signal from the MZM, and
modulate the first modulated signal to produce a second modulated
signal according to the second electrical data signal; and a
controlling circuit coupled to the PM, the controlling circuit
configured to adjust a direct current (DC) bias of the PM, the
controlling circuit configured to adjust a first DC bias of the
first PM segment separately from a second DC bias of the second PM
segment.
5. The apparatus of claim 1, wherein the MZM further comprises n
number of MZM modulator segments, wherein the second modulated
signal is an n-level quadrature amplitude modulation (nQAM) signal,
and wherein n is a number greater than one.
6. The apparatus of claim 4, wherein the MZM comprises two MZM
modulator segments, and wherein the second modulated signal is a
16-level quadrature amplitude modulation (16QAM) signal.
Description
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
REFERENCE TO A MICROFICHE APPENDIX
Not applicable.
BACKGROUND
In metro and short-reach optical networks, such as inter- and
intra-data centers, low power consumption, low cost, and high
density are three important factors for optoelectronics components.
However, optical networks commonly employ a large number of optical
transceivers, which are the most expensive components and consume
the most power. Optical transceivers may include components, such
as high-speed analog-to-digital converters (ADCs), high-speed
digital-to-analog converters (DACs), optical modulators, and radio
frequency (RF) drivers. In order to meet the higher and higher
bandwidth requirements due to the exponential growth of Internet
traffic, advanced modulation formats that are used in optical long
haul transmission systems, such as direct detection-differential
quadrature phase-shift keying (DD-DQPSK), dual-polarization
quadrature phase-shift keying (DP-QPSK), and 16 quadrature
amplitude modulation (16QAM), are also deployed in short-reach
networks. However, most of the optical components used in long haul
transmission systems may not satisfy the low power consumption, low
cost, and high density requirements in short-reach and metro
optical networks.
Most of the advanced modulation formats in commercial optical
equipment are based on Mach-Zehnder modulators (MZMs). The working
principle of MZMs is to modulate the optical phase difference
between two waveguides, which then interfere constructively or
destructively to achieve amplitude modulation and phase modulation
on the output. For example, MZMs are employed to generate highly
stable optical signals as described in H. Kiuchi, et. al., "High
Extinction Ratio Mach-Zehnder Modulator Applied to a Highly Stable
Optical Signal Generator," Institute of Electronics and Electrical
Engineers (IEEE) Transactions of Microwave Theory and Techniques,
Vol. 55, No. 9, September 2007, pp. 1694-1972, which is
incorporated by reference. MZMs that are commonly employed in
industry may include lithium nobiate (LiNbO.sub.3)-based MZMs,
indium phosphide (InP)-based MZMs, and silicon (Si)-based MZMs. Due
to the high-density requirement in optical transmitters,
LiNbO.sub.3-based transmitters may not be suitable for short-reach
applications. The highly integrated Si-based transmitters and
InP-based transmitters are more suitable for short-reach
applications.
FIG. 1 is a schematic diagram of a conventional optical in-phase
quadrature-phase quadrature phase-shift keying (IQ QPSK) modulator
100 which modulates an input signal 20 emitted by laser diode (LD)
10 according to a digital signal to produce a modulated output
signal 30. Modulator 100 comprises three MZMs 110, 120, and 130.
MZMs 120 and 130 are referred to as child modulators and MZM 110 is
referred to as a parent modulator. MZMs 120 and 130 are positioned
in parallel with each other. MZM 120 is configured to generate
in-phase (I) components according to RF driver 121. MZM 130 is
configured to generate quadrature-phase (Q) components according to
RF driver 131. The output of MZM 130 passes through phase shifter
133, and output signals 122 and 132 are combined, resulting in
modulated output data signal 30. MZMs 120 and 130 operate at null
points, which are transmission minimum points, and MZM 110 operates
at a quadrature point, which is a 3 decibel (dB) loss point. FIGS.
2A-2C illustrate the output signals, in the form of constellation
diagrams, as generated by the MZMs of IQ QPSK modulator 100. FIG.
2A illustrates a constellation diagram of output signal 122 of MZM
120. FIG. 2B illustrates a constellation diagram of output signal
132 of MZM 130. FIG. 2C illustrates a constellation diagram of an
output signal 30 of modulator 100.
Modulator 100 may be employed to generate 16QAM and higher order
modulation signals by configuring RF drivers 121 and 131 to
generate multi-level outputs as described in T. Sakamoto, et. al.,
"50-Gb/s 16 QAM by a Quad-Parallel Mach-Zehnder Modulator," Optical
Communication--Post Deadline Papers, 33rd European Conference,
2007, pp. 1-2, which is incorporated by reference. Modulator 100
may also be employed to generate 16QAM and higher order modulation
signals by cascading modulator 100 and an optical phase modulator
(PM) as described in M. Serbay, et. al., "Implementation of
Differential Precoder for High-Speed Optical DQPSK Transmission,"
Electronics Letters, volume 40, issue 20, 30 Sep. 2004, pp.
1288-1289 (Serbay), which is incorporated by reference.
SUMMARY
One embodiment is a method for using a phase modulator (PM) to
generate a phase-shift keying (PSK) data signal, the method
comprising receiving an optical input signal; coupling with a
direct current (DC) bias signal from a controlling circuit;
adjusting the modulation efficiency of the PM by controlling the DC
bias signal; coupling with a digital data signal from a precoder;
and modulating the input signal according to the digital data
signal to produce the PSK data signal.
A variation on this embodiment is wherein the PM comprises a first
PM segment coupled with a second PM segment, and wherein adjusting
the modulation efficiency of the PM comprises adjusting a DC bias
of the first PM segment separately from a DC bias of the second PM
segment. Another variation on this embodiment is wherein the input
signal is a continuous wave signal. Another variation on this
embodiment is wherein the modulated data signal is a second
modulated signal, and wherein the input signal is a first modulated
signal which may be generated by a Mach-Zehnder modulator or by
another PM.
Another embodiment is an apparatus for generating an optical
phase-shift keying (PSK) data signal from an optical input signal,
the apparatus comprising a multi-segment doped waveguide configured
to receive the optical input signal and to output the PSK data
signal; a controlling circuit coupled with the multi-segment
waveguide, wherein the controlling circuit is configured to adjust
the modulation efficiency of the PM by controlling the direct
current (DC) bias signal of each segment of the waveguide; a
plurality of inverter drivers coupled with the waveguide; and a
precoder coupled with the plurality of inverter drivers, wherein
the precoder is configured to provide a plurality of digital data
signals to the plurality of inverter drivers.
A variation on this embodiment is to add a laser diode coupled with
the waveguide, wherein the laser diode provides the input optical
signal. Another variation on this embodiment is where a phase
modulator (PM) coupled with the waveguide provides the input
optical signal. Another variation on this embodiment is where a
Mach-Zehnder modulator (MZM) coupled with the waveguide provides
the input optical signal.
Another embodiment is an apparatus for generating a modulated data
signal, the apparatus comprising a precoder configured to code data
for modulation and outputting a plurality of high-speed data
signals; a Mach-Zehnder modulator (MZM) coupled to the precoder,
wherein the MZM is configured to receive a first data signal of the
plurality of data signals and to modulate an input signal to
produce a first modulated signal according to the first data
signal; a phase modulator (PM) coupled to the precoder and to the
MZM, wherein the PM is configured to receive a second data signal
of the plurality of data signals, and wherein the PM is configured
to receive the first modulated signal from the MZM to produce a
second modulated signal according to the second data signal; and a
controlling circuit coupled to the PM, wherein the controlling
circuit is configured to adjust a direct current (DC) bias of the
PM.
One variation on this embodiment is to add a laser diode configured
to transmit the input signal, wherein the laser diode is coupled
with the MZM. Another variation on this embodiment is the MZM
generating 0 and .pi. phase changes and with the PM generating 0
and +.pi./2 phase changes, wherein the second modulated signal is a
4-level quadrature amplitude modulation (QAM) signal. Another
variation on this embodiment is wherein the MZM and PM comprise
silicon photonics (SiP) materials. Another variation on this
embodiment is wherein the PM further comprises a first PM segment
coupled with a second PM segment, and wherein the controlling
circuit adjusts a DC bias of the first PM segment separately from a
DC bias of the second PM segment. Another variation on this
embodiment is wherein the MZM further comprises n MZM modulator
segments, wherein the second output signal is an n-level quadrature
amplitude modulation (QAM) (nQAM) signal, and wherein n is a number
greater than one. Another variation on this embodiment is wherein
the PM further comprises a first PM segment coupled with a second
PM segment, wherein the controlling circuit adjusts a DC bias of
the first PM segment separately from a DC bias of the second PM
segment, wherein the MZM comprises two MZM modulator segments, and
wherein the second output signal is a 16QAM signal.
Another embodiment is an apparatus for generating a dual
polarization modulated data signal, the apparatus comprising an
optical splitter configured to split a continuous wave input signal
into an X-polarization path and a Y-polarization path; a first
Mach-Zehnder modulator-phase modulator (MZM-PM) modulator and a
second MZM-PM modulator, the first and second MZM-PM modulators
each comprising a Mach-Zehnder modulator (MZM) configured to
modulate an input signal to produce a first modulated signal; a
phase modulator (PM) configured to modulate the first modulated
signal to produce a second modulated signal; a precoder configured
to code data for modulation by the MZM and PM; and a controlling
circuit configured to adjust a direct current (DC) bias of the PM;
a polarization rotator coupled with the second MZM-PM modulator;
and a polarization beam combiner coupled with first MZM-PM and the
polarization rotator, wherein the X-polarization path is the input
signal for the first MZM-PM and wherein the Y-polarization path is
the input signal for the second MZM-PM.
One variation on this embodiment is to add a laser diode to
transmit the continuous wave input signal. Another variation on
this embodiment is wherein the MZM of the first MZM-PM is
configured to generate 0 and .pi. phase changes, wherein the PM of
the first MZM-PM is configured to generate 0 and +.pi./2 phase
changes, and wherein the second modulated signal of the first
MZM-PM is a 4-level quadrature amplitude modulation (QAM) signal.
Another variation on this embodiment is wherein the first MZM-PM
comprises silicon photonics (SiP) materials. Another variation on
this embodiment is wherein the PM of the first MZM-PM comprises a
first PM segment coupled with a second PM segment, and wherein the
controlling circuit of the first MZM-PM adjusts a DC bias of the
first PM segment separately from a DC bias of the second PM
segment. Another variation on this embodiment is wherein the MZM of
the first MZM-PM further comprises n MZM modulator segments,
wherein the second output signal of the first MZM-PM is an n-level
quadrature amplitude modulation (QAM) (nQAM) signal, and wherein n
is a number greater than 1. Another variation on this embodiment is
wherein the PM of the first MZM-PM comprises a first PM segment
coupled with a second PM segment, wherein the controlling circuit
of the first MZM-PM adjusts a DC bias of the first PM segment
separately from a DC bias of the second PM segment, wherein the MZM
of the first MZM-PM comprises 2 segments of modulators, and wherein
the second output signal of the first MZM-PM is a 16QAM signal.
These and other features will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of this disclosure, reference is
now made to the following brief description, taken in connection
with the accompanying drawings and detailed description, wherein
like reference numerals represent like parts.
FIG. 1 is a schematic diagram of a conventional IQ QPSK
modulator.
FIGS. 2A-2C illustrate constellations generated by the conventional
IQ QPSK modulator of FIG. 1.
FIG. 3 is a schematic diagram of a Mach-Zehnder modulator phase
modulator (MZM-PM) configuration that implements modulation
efficiency control according to an embodiment of the
disclosure.
FIGS. 4A and 4B illustrate constellations generated by employing
the MZM-PM configuration of FIG. 3.
FIG. 5 is a graph illustrating positive-negative (PN) junction
capacitance of a Si-based phase modulator as a function of DC bias
voltages.
FIG. 6 is a demodulated eye diagram of a 28 gigabaud (Gbaud)
differential quadrature phase-shift keying (DQPSK) signal generated
by a prior art IQ DQPSK modulator.
FIG. 7 is a demodulated eye diagram of a 28 Gbaud DQPSK signal
generated by employing the MZM-PM configuration of FIG. 3.
FIG. 8 is a graph comparing bit-error-rate (BER) performances of a
28 Gbaud DQPSK signal generated by employing the MZM-PM
configuration of FIG. 3 with that of a prior art IQ modulator.
FIG. 9 is a graph illustrating receiver sensitivity as a function
of PM driving swing ratios at various transmitter (Tx)
bandwidths.
FIG. 10 is a schematic diagram of an nQAM modulator that implements
modulation efficiency control according to an embodiment of the
disclosure.
FIGS. 11A and 11B illustrate constellations generated by the nQAM
modulator of FIG. 10.
FIG. 12 is a schematic diagram illustrating a scheme for
controlling time delay between optical and electrical signals
according to an embodiment of the disclosure.
FIG. 13 is a schematic diagram of a dual-polarization 16 quadrature
amplitude modulation (DP-16QAM) modulator that implements
modulation efficiency control according to an embodiment of the
disclosure.
FIG. 14 is a constellation diagram of a 12QAM signal generated by
configuring the nQAM modulator of FIG. 10 as a 16QAM modulator.
FIG. 15 is a constellation diagram of a star 8QAM signal generated
by configuring the nQAM modulator of FIG. 10 as a 16QAM
modulator.
FIG. 16 is a schematic diagram of a DP-16QAM modulator that
implements modulation efficiency control according to another
embodiment of the disclosure.
FIG. 17 is a schematic diagram of an nPSK modulator according to an
embodiment of the disclosure.
DETAILED DESCRIPTION
It should be understood at the outset that, although illustrative
implementations of one or more embodiments are provided below, the
disclosed systems and/or methods may be implemented using any
number of techniques, whether currently known or in existence. The
disclosure should in no way be limited to the illustrative
implementations, drawings, and techniques illustrated below,
including the exemplary designs and implementations illustrated and
described herein, but may be modified within the scope of the
appended claims along with their full scope of equivalents.
Disclosed herein are various embodiments for generating nQAM and
nPSK signals with tunable modulation efficiency. The disclosed
embodiments provide a method and apparatus to reduce coherent
transceiver power consumption and cost. The disclosed embodiments
also improve flexibility in generation of different modulation
formats. The disclosed embodiments employ silicon photonics
(SiP)-based optical modulators to generate nQAM and nPSK
signals.
Prior art QPSK modulators and QAM modulators produce a signal
having an intrinsic 3 dB optical loss because MZM 110 needs to
operate at a quadrature point. In addition, prior art MZMs need to
be locked to the quadrature point, which adds complexity to the
control of the prior art modulator and increases power-on time.
Further, while it is known to drive a prior art PM as a function of
RF signal amplitudes to produce phase modulation, the performance
of the PM may be degraded by RF signal variations at the beginning
of life and after aging.
To resolve these and other problems, and as will be more fully
explained below, embodiments of the apparatuses and methods
disclosed herein generate n-level quadrature amplitude modulation
(nQAM) and n-level phase-shift keying (nPSK) signals with tunable
modulation efficiency that does not incur the intrinsic 3 dB
optical loss. The embodiments discussed herein employ a simpler
lock algorithm. The embodiments discussed herein are stable over
temperature and aging. The embodiments discussed herein operate as
a function of DC biases instead of RF signal amplitudes.
In an embodiment, an optical modulator comprises an MZM coupled to
an optical PM, where the modulation efficiency or modulation depth
of the PM is adjusted by controlling the DC bias of the PM. To
generate nQAM signals, the optical modulator may employ a segmented
MZM with n plurality of sections of modulators. In another
embodiment, an optical modulator comprises n plurality of cascading
segments of PMs configured to produce an nPSK signal, where the
modulation efficiency of the PMs is tuned by controlling the DC
biases of the PMs. The disclosed embodiments do not experience a 3
dB modulation loss. The disclosed embodiments employ a
significantly simpler lock algorithm. The disclosed SiP-based
optical modulators are stable over temperature and aging. The
disclosed PM operates as a function of DC biases instead of RF
signal amplitudes. The disclosed modulation efficiency control
mechanisms can applied to any type of optical modulators, such as
DP-16QAM modulators, DP-8QAM modulators, DP-12QAM modulators, and
nPSK modulators.
FIG. 3 discloses a schematic diagram of MZM-PM modulator 300 which
modulates input signal 320 from light source 310 to produce nQAM
output signal 330 using tunable modulation efficiency. Modulator
300 may be employed by an optical transmitter to modulate data
signals for transmission. Modulator 300 may be constructed from
SiP-based materials in some embodiments. Modulator 300 comprises
MZM 340, PM 350, and controlling circuit 360. MZM 340 is coupled to
light source 310 and configured to generate 0 and .pi. phase
changes (as a result of modulation) when MZM 340 operates at a null
point. PM 350 is coupled to MZM 340 and receives the modulated
light signal that is modulated by MZM 340, wherein PM 350 is
configured to generate 0 and +.pi./2 phases in addition to the
phase changes generated by MZM 340. High-speed electrical data
signals 371.sub.1 and 371.sub.2 from precoder 370 control the
modulations performed by MZM 340 and PM 350 respectively. Thus,
modulator 300 produces a 4-level QPSK signal 330 in the example
shown. The controlling circuit 360 is configured to control the DC
bias 361 of the PM 350. By tapping PM 350's output optical power to
controlling circuit 360, controlling circuit 360 may process data
according to the output optical power and adjust the DC bias
voltage 361 of PM 350 such that DC bias voltage 361 is locked to an
optimum point. Controlling circuit 360 and precoder 370 may be
implemented using discrete circuitry, combined on an
application-specific integrated circuit, or using any other
configuration known to one of ordinary skill.
FIGS. 4A and 4B disclose the output signals generated by employing
modulator 300. FIG. 4A is a constellation diagram of output signal
341 of MZM 340 showing 0 and .pi. phase changes produced by MZM
340. FIG. 4B is a constellation diagram of output signal 330 at the
output of PM 350 showing the 0 and +.pi./2 phases produced by PM
350 (in addition to the 0 and .pi. phase changes produced by MZM
340).
FIG. 5 discloses a graph illustrating the PN junction capacitance
of SiP-based PM 350 as a function of DC bias voltages 361. The
x-axis represents DC bias voltage 361, shown as anode bias, in
units of Volts. The y-axis represents PN junction capacitance,
shown as cathode-anode capacitance, in units of Farads. As shown,
the PN junction capacitance is high at low bias and is much higher
at a slightly forward bias condition. The PN junction capacitance
corresponds to modulation efficiency. The capacitance of the PN
junction can be increased greatly when it is biased at a slight
forward voltage. Thus, the phase modulation depth of PM 350 may be
adjusted by controlling DC bias voltage 361 of PM 350. The phase
modulation depth (sometimes referred to as phase modulation index)
refers to how much the phase modulated variable of the carrier
signal varies around its unmodulated level. Therefore, the phase
modulation depth relates to the variation in the phase of the
carrier signal.
FIG. 6 discloses a demodulated eye diagram of a 28 Gbaud DQPSK
signal generated by a prior art IQ modulator, and FIG. 7 is a
demodulated eye diagram of a 28 Gbaud DQPSK signal generated by
employing modulator 300. As can be seen by comparing FIGS. 6 and 7,
MZM-PM modulator 300 produces similar results as the prior art
DQPSK modulator. FIG. 8 discloses a graph comparing bit error rate
(BER) as a function of received optical power (ROP) of a 28 Gbaud
DQPSK signal 801 generated by a prior art IQ modulator with that of
a 28 Gbaud DQPSK signal 802 generated by employing modulator 300.
As can be seen by comparing signals 801 and 802, MZM-PM modulator
300 produces similar results as the prior art DQPSK modulator.
FIG. 9 discloses a graph illustrating receiver sensitivity as a
function of driving swing ratios of PM 360 to pull-in voltage (Vpi)
at various transmitter bandwidths 901 (12 gigahertz (GHz)), 902 (15
GHz), 903 (18 GHz), and 904 (21 GHz). For example, a transmitter
employing MZM-PM modulator 300 is configured to transmit a signal
and the receiver sensitivity is measured at a receiver, wherein the
receiver is configured to receive the signal from the
transmitter.
For a given transmitter bandwidth, there is an optimum driving
ratio. The performance of a bandwidth limited transmitter may be
improved by over-driving PM 350. The driving swing ratio of PM 350
may be varied by adjusting DC bias 361 of PM 350. Thus, the
performance may be tuned by adjusting the DC bias 361 of PM 350
such that the receiver sensitivity is optimized for a given
transmitter bandwidth.
Alternatively, the required phase modulation depth may be adjusted
to a desired point by controlling the DC bias 361 of PM 350. The
tunable phase efficiency mechanisms may also be applied to
compensate Vpi variations due to the SiP MZM fabrication process,
and thus may improve yields of SiP-based optical transmitters.
FIG. 10 discloses a schematic diagram of MZM-PM modulator 1000
which modulates input signal 1020 from light source 1010 to produce
QAM output signal 1030 using tunable modulation efficiency.
Modulator 1000 may be employed by an optical transmitter to
modulate data signals for transmission and may be constructed from
SiP-based materials. Modulator 1000 comprises segmented MZM 1040
and segmented PM 1050. Segmented MZM 1040 comprises a similar
configuration as described in Y. Ehrlichman, et. al, "Improved
Digital-to-Analog Conversion Using Multi-Electrode Mach-Zehnder
Interferometer," Journal of Lightwave Technology, vol. 26, no. 21,
Nov. 1, 2008 and Zheng, et. al, "Digital Optical Modulator for
Programmable nQAM Generation," and in United States Patent
Publication US2015/0132007A1, both of which are incorporated by
reference. Segmented MZM 1040 comprises n segments of modulators,
driven by electrical high-speed data signals 1071.sub.1 through
1071.sub.n from precoder 1070. The two segments of segmented PM
1050 are driven by electrical high-speed data signals 1071.sub.n+1
and 1071.sub.n+2 Segmented PM 1050 is thus configured to generate
additional phases such as 0, .pi./4, .pi./2, or 3.pi./4, which are
added to the signal generated by the segmented MZM 1040. In other
embodiments, segmented PM 1050 may have additional segments to
produce additional phases. Controlling circuit 1060 controls DC
biases 1061.sub.1 and 1061.sub.2 of segmented PM 1050 to optimize
the modulation efficiency of segmented PM 1050. Like modulator 300,
controlling circuit 1060 and precoder 1070 may be implemented using
discrete circuitry, combined on an application-specific integrated
circuit, or using any other configuration known to one of ordinary
skill.
Using the configuration shown in FIG. 10, modulator 1000 avoids the
intrinsic 3 dB modulation loss that is found in conventional 16QAM
modulators. In addition, modulator 1000 eliminates one pair of MZMs
and the corresponding RF drivers when compared to the configuration
of conventional 16QAM modulators.
FIGS. 11A and 11B disclose the output signals generated by
employing modulator 1000. FIG. 11A is a constellation diagram of
output signal 1041 of segmented MZM 1040 when there are two
segments (i.e., when n=2). FIG. 11B is a constellation diagram of
the 16QAM output signal 1030 when there are two segments in
segmented MZM 1040 (i.e., when n=2).
FIG. 12 discloses a schematic diagram illustrating an embodiment of
a portion of a segmented MZM 1200 with improvements for controlling
time delay between optical and electrical signals. MZM 1200
receives an optical input signal 1220 and produces a modulated
optical output signal 1230. Segmented MZM 1200 comprises n segments
1240.sub.n, each receiving an electrical high-speed data signal
1244.sub.n, logical inverter driver 1242.sub.n, and a doped
waveguide portion 1241.sub.n. MZM segments 1240.sub.2 through
1240.sub.n also include delays 1243.sub.2 through 1243.sub.n. The
delay of the optical signal between waveguide portions 1241.sub.1
and 1241.sub.2 is .DELTA.t, and the delay of the electrical signal
created by delay 1243.sub.2 is also .DELTA.t. Similarly delays
1243.sub.3 through 1243.sub.n create electrical delays equal to the
cumulative optical delays in the chain of waveguide portions
1241.sub.3-1241.sub.n so that the input data signals are similarly
delayed and are therefore in-phase with the optical signal in each
waveguide portion. Thus the optical delay between 1241.sub.1 and
1241.sub.n and the delay created by delay 1243.sub.n are both
.DELTA.t.sub.n. The configuration shown in FIG. 12 can be applied
to the MZMs any of the embodiments of the present disclosure.
FIG. 13 discloses a schematic diagram of a DP-16QAM modulator 1300
that uses tunable modulation efficiency according to an embodiment
of the disclosure. The modulator 1300 is constructed by duplicating
the structure of MZM-PM modulator 1000 as parallel MZM-PM 1325 and
MZM-PM 1326. The modulator 1300 splits a continuous optical wave
input signal 1320 into two signals 1321 and 1322, referred to as an
X-polarization path and a Y-polarization path, respectively. Input
signals 1321 and 1322 pass through MZM-PM 1325 and MZM-PM 1326,
respectively. Like segmented MZM 1040 in FIG. 10, each segmented
MZM 1340 comprises n segments of modulators driven by electrical
high-speed data signals 1371.sub.1 through 1371.sub.n from precoder
1370, and produces output signals 1341. Like segmented PM 1050 in
FIG. 10, each segmented PM 1350 comprises two segments driven by
electrical high-speed data signals 1371.sub.n+1 and 1371.sub.n+2.
Controlling circuits 1360 adjust the DC biases 1361.sub.1 and
1361.sub.2. The output signal of MZM-PM 1326 passes through
polarization rotator 1380, which rotates the signal 90.degree., and
modulated signals from MZM-PM 1325 and polarization rotator 1380
path are rejoined by polarization beam combiner 1390 to produce
output signal 1330. Each segmented MZM 1340 produces
one-dimensional amplitude and phase modulated signals. For example,
when each segmented MZM 1340 comprises two segments (i.e., n=2),
each segmented MZM 1340 produces an output signal similar to the
constellation diagram shown in FIG. 11A. Each segmented PM 1350
produces an output signal similar to the constellation diagram
shown in FIG. 11B.
In an embodiment, the modulator 1300 with a two-segment MZM may be
extended to generate 12QAM and star 8QAM by manipulating the
electrical high-speed data signals 1371.sub.n in the logic
functions of precoders 1370. FIG. 14 is a constellation diagram of
a 12QAM signal generated by configuring modulator 1300 as a 16QAM
modulator with a two-segment MZM. The 12QAM modulated signal is
generated by removing four points from the inner ring of the 16QAM
constellation shown in the constellation diagram of FIG. 11B and
rotating the eight points in the outer ring by .pi./8. A 12QAM
modulated signal carries 7 bits for every 2 symbols, where the
mapping may be realized in the precoder 1370 via logic operation.
As shown, the 12QAM constellation has equal spacing at the outer
ring which is expected to provide better performance than
conventional 12QAM.
FIG. 15 discloses a constellation diagram of a star 8QAM signal
generated by configuring nQAM modulator 1000 as a 16QAM modulator
with two MZM segments by manipulating electrical high-speed data
signals 1371.sub.n in the logic functions of precoders 1370, and
using two MZM segments. The star 8QAM modulated signal is generated
by removing four points from the outer ring and four points from
the inner ring of the 16QAM constellation shown in the
constellation diagram in FIG. 11B and rotating the remaining four
points in the inner ring by .pi./4. The mapping of the star 8QAM
may be realized in the precoder 1370 via a logic operation.
Further, by manipulating electrical high-speed data signals
1371.sub.n in the logic functions of precoders 1370 in a
configuration with two MZM segments, modulator 1300 can be modified
to generate DP-12QAM and star DP-8QAM with similar process to those
of single polarization.
FIG. 16 discloses a schematic diagram of DP-16QAM modulator 1600
which uses tunable modulation efficiency. Modulator 1600 splits
optical input signal 1620 into input signal 1621 on the
X-polarization path and input signal 1622 on the Y-polarization
path. MZMs 1640 are driven by 4-level electrical high-speed data
signals 1671.sub.1 and segmented PMs 1650 are driven by 2-level
electrical high-speed data signals 1671.sub.2 and 1671.sub.3. The
modulation efficiency of modulator 1600 is adjusted by controlling
circuits 1660 by adjusting DC biases 1661 of PMs 1650. Modulator
1600 further employs precoders 1670 to code and synchronize the
electrical signals and the optical signals. The output signal of
MZM-PM 1626 passes through polarization rotator 1680, which rotates
the signal 90.degree., and modulated signals from MZM-PM 1625 and
polarization rotator 1680 path are rejoined by polarization beam
combiner 1690 to produce output signal 1630. Each MZM 1640 produces
one-dimensional amplitude and phase modulated signals 1641
comprising constellations similar to the constellation diagram
shown in FIG. 11A. Output 1630 produces multi-levels constellation
similar to constellation diagram FIG. 11B.
FIG. 17 discloses a schematic diagram of an nPSK modulator 1700
according to an embodiment of the disclosure. The modulator 1700
comprises multi-segment PM 1750 controlled by precoder 1770 to
produce modulated output signal 1730. Precoder 1770 produces
electrical high-speed data signals 1744.sub.n which are inverted by
inverter drivers 1742.sub.n and coupled with PM segments
1750.sub.n. The modulation efficiency of the segments of PM 1750 is
tuned using controlling circuit 1760 by adding DC biases 1761.sub.n
to corresponding segments of PM 1750. Input signal 1720 may be a
continuous wave signal or it may be the modulated output signal of
an MZM; for the latter, nPSK modulator 1700 can be used for PM 350,
PM 1050, PM 1350, or PM 1650. The advantages of the disclosed nPSK
modulator 1700 compared to the prior art modulators may include
tunable modulation efficiency, simplified PM bias control,
significant RF power consumption saving, significant optical loss
saving, for example, about 3 dB, and significant size reduction due
to the employment of fewer Mach-Zehnder interferometers (MMIs) and
MZMs.
Disclosed herein in is a method for using a phase modulator (PM) to
generate a phase-shift keying (PSK) data signal. The method
includes means for receiving an optical input signal, means for
coupling with a direct current (DC) bias signal from a controlling
circuit, means for adjusting the modulation efficiency of the PM by
controlling the DC bias signal, means for coupling with a digital
data signal from a precoder, and means for modulating the input
signal according to the digital data signal to produce the PSK data
signal.
Further disclosed herein is an apparatus for generating an optical
phase-shift keying (PSK) data signal from an optical input signal.
The apparatus includes a means for receiving the optical input
signal and to output the PSK data signal; a means for adjusting the
modulation efficiency of the PM by controlling the direct current
(DC) bias signal of each segment of the waveguide, and a means for
providing a plurality of digital data signals to the plurality of
inverter drivers.
Further disclosed herein is an apparatus for generating a modulated
data signal. The apparatus includes a means for coding data for
modulation and outputting a plurality of high-speed data signals,
and means for receiving a first data signal of the plurality of
data signals and modulating an input signal to produce a first
modulated signal according to the first data signal, a means for
receiving a second data signal of the plurality of data signals, a
means for receiving the first modulated signal from the MZM to
produce a second modulated signal according to the second data
signal, and a means for adjusting a direct current (DC) bias of the
PM.
Further disclosed herein is an apparatus for generating a dual
polarization modulated data signal. The apparatus includes a means
for splitting a continuous wave input signal into an X-polarization
path and a Y-polarization path, a means for modulating an input
signal to produce a first modulated signal, a means for modulating
the first modulated signal to produce a second modulated signal, a
means for coding data for modulation by the MZM and PM, a means for
adjusting a direct current (DC) bias of the PM, a means for
rotating the signal from the second MZM-PM modulator, and a means
for combining the signals from the first and second MZM-PMs.
While several embodiments have been provided in the present
disclosure, it may be understood that the disclosed systems and
methods might be embodied in many other specific forms without
departing from the spirit or scope of the present disclosure. The
present examples are to be considered as illustrative and not
restrictive, and the intention is not to be limited to the details
given herein. For example, the various elements or components may
be combined or integrated in another system or certain features may
be omitted, or not implemented.
In addition, techniques, systems, subsystems, and methods described
and illustrated in the various embodiments as discrete or separate
may be combined or integrated with other systems, modules,
techniques, or methods without departing from the scope of the
present disclosure. Other items shown or discussed as coupled or
directly coupled or communicating with each other may be indirectly
coupled or communicating through some interface, device, or
intermediate component whether electrically, mechanically, or
otherwise. Other examples of changes, substitutions, and
alterations are ascertainable by one skilled in the art and may be
made without departing from the spirit and scope disclosed
herein.
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